Iran confirmed it owned the tanker that was detained off Gibraltar on 4 July when it accused the United Kingdom of carrying out an act of piracy in international waters.
"Britain has no right to impose its own unilateral sanctions or those of the European Union [EU] in an extraterritorial manner," the Iranian Foreign Ministry said in a statement.
The seizure of the Panama-flagged Grace 1 early on 4 July was announced by Chief Minister of Gibraltar Fabian Picardo. He said the tanker was boarded by the British territory's police force with assistance from a detachment of Royal Marines because it was believed to be carrying oil to Syria's Baniyas refinery in contravention of EU sanctions.
UK Defence Secretary Penny Mordaunt praised the "fantastic work" of Royal Marines who "used their specialist skills to board a cargo ship to enforce sanctions on Syria".
Hinting at United States involvement, US National Security Adviser John Bolton praised the seizure, posting on Twitter that "America and our allies will continue to prevent regimes in Tehran and Damascus from profiting off this illicit trade."
Automatic Identification System (AIS) transponder data showed the tanker may have been loaded at Iran's Khargh island terminal in April, before sailing around Africa. Large tankers carrying oil for Mediterranean countries normally make a much shorter journey through the Red Sea to unload at the Sumed pipeline that crosses Egypt.
The AFP reported that the tanker was boarded 4 km south of Gibraltar as it slowed in a designated area used by shipping agencies to ferry supplies to vessels. That would have put it inside the territorial waters claimed by the UK but disputed by Spain.
Want to read more? For analysis on this article and access to all our insight content, please enquire about our subscription options at ihsmarkit.com/janes